3.6 Job Specification: The $rem Array Concept
The $rem array is the means by which users convey to Q-Chem the type of calculation they wish to perform (level of theory, basis set, convergence criteria, etc.). The keyword $rem signals the beginning of the overall job specification. Within the $rem section the user inserts $rem variables (one per line) which define the essential details of the calculation. The format for entering $rem variables within the $rem keyword section of the input is shown in the following example shown in the following example:
Example 3.13 Format for declaring $rem variables in the $rem keyword section of the Q-Chem input file. Note, Q-Chem only reads the first two arguments on each line of $rem. All other text is ignored and can be used for placing short user comments.
REM_VARIABLE VALUE [comment]
The $rem array stores all details required to perform the calculation, and details of output requirements. It provides the flexibility to customize a calculation to specific user requirements. If a default $rem variable setting is indicated in this manual, the user does not have to declare the variable in order for the default to be initiated (e.g., the default JOBTYPE is a single point energy, SP). Thus, to perform a single point energy calculation, the user does not need to set the $rem variable JOBTYPE to SP. However, to perform an optimization, for example, it is necessary to override the program default by setting JOBTYPE to OPT.
A number of the $rem variables have been set aside for internal program use, as they represent variables automatically determined by Q-Chem (e.g., the number of atoms, the number of basis functions). These need not concern the user.
User communication to the internal program $rem array comes in two general forms: (1) long term, machine-specific customization via the .qchemrc and preferences files (Section 2.6) and, (2) the Q-Chem input deck. There are many defaults already set within the Q-Chem program many of which can be overridden by the user. Checks are made to ensure that the user specifications are permissible (e.g. integral accuracy is confined to 
and adjusted, if necessary. If adjustment is not possible, an error message is returned. Details of these checks and defaults will be given as they arise.
The user need not know all elements, options and details of the $rem array in order to fully exploit the Q-Chem program. Many of the necessary elements and options are determined automatically by the program, or the optimized default parameters, supplied according to the user’s basic requirements, available disk and memory, and the operating system and platform.